1. Field of the Invention
This invention relates to a method for producing a green article, more particularly to a method for producing a three dimensional green article.
2. Description of the Related Art
Conventional methods for producing a three dimensional green article include a press-and-sinter fabrication (in which applying a high pressure is required), a powder injection molding process (in which applying a high pressure and adding a binder are required), and a slip casting process (in which a binder is used but application of pressure is not required). The step of applying a pressure requires a machine for producing pressure and a high-pressure-resistant die. In contrast, the process without pressure application merely requires a low pressure-resistant die (for example, a gypsum die for slip casting).
Recently, a rapid prototyping technology has been developed and is characterized by producing a three dimensional workpiece using a stacking principle. In general, the rapid prototyping technology involves moving a tool along a complicated two dimensional work path based on layer-slicing data stored in a computer to obtain a plurality of green layers, followed by stacking and interconnecting the green layers to form a three dimensional workpiece. The feature of such technology resides in that an extremely complicated shape of a workpiece can be made without using a die. Because no die is required, the process is a manufacturing method without pressure application, and thus, a binder is required to bind powder particles to form a green article. Thereafter, a step of densification sintering is performed to produce a workpiece having a desired strength.
In the rapid prototyping technology, the work path can be created using different tools, for example, a nozzle for a three dimensional printing (3DP) process, an extrusion head for a fused deposition modeling (FDM) process, and an energy beam for selective laser sintering (SLS) and stereo-lithography (SLA) processes. The process for producing a three dimensional green article using an energy beam may be classified into an additive process and a subtractive process. The feature of the additive process resides in that a part of the green layer that is scanned by the energy beam is bound to form a portion of the green article. Examples of the additive processes include SLA, SLS, and slurry based selective laser sintering processes. The feature of the subtractive process resides in that a part of the green layer that is scanned by the energy beam is removed and the other part that is not scanned by the energy beam becomes a portion of the green article. Examples of the subtractive processes include laminated object manufacturing (LOM) process, computer-aided manufacturing of laminated engineering material (CAM-LEM) process and the like.
Referring to FIG. 1, in US Patent Application Publication No. 2004/0075197, the applicant of this invention discloses a method for producing a green article 17, the method being based on the slurry based selective laser sintering process, and including following steps:
(A) mixing ceramic powder, a dissolving agent and oxide sol together to form a slurry composition 11;
(B) placing the slurry composition 11 on a platform 12 and moving a blade 13 along a horizontal direction (X) to pave the slurry composition 11 on the platform 12, thereby forming a slurry layer 14;
(C) irradiating the slurry layer 14 along a planar (i.e., two dimensional) predetermined scanning path using a laser beam 15, so that the oxide sol in the slurry layer 14 that is irradiated by the laser beam 15 is gelled to bind the ceramic powder, thereby forming a two dimensional ceramic green layer 16;
(D) repeating steps (B) and (C) for a predetermined number of times to form a plurality of green layers 16, followed by binding the green layers 16 using the laser beam 15 to form a three dimensional ceramic green article 17; and
(E) after step (D), immersing the three dimensional ceramic green article 17 in water 18 to dissolve the un-gelled oxide sol, thereby obtaining the three dimensional ceramic green article 17.
In the method disclosed in US 2004/0075197, the slurry layer 14 is irradiated along a planar (i.e., two dimensional) predetermined scanning path in step (C). In particular, it is noted that the larger the volume of the finished ceramic green article 17, the larger will be the scanning area defined by the planar scanning path. Therefore, the work time for forming the finished ceramic green article 17 is expected to be relatively long.
From the above, it is desired to reduce the work time for producing a three dimensional ceramic green article so as to reduce costs for producing the three dimensional ceramic green article, thereby providing cost-effective advantage for the technology used to produce a large workpiece.
When a three dimensional workpiece is produced by LOM or CAM-LEM of the subtractive process, the linear workpiece profile is scanned by a laser, thereby saving the work time. However, the LOM process disclosed in U.S. Pat. No. 4,752,352 is suitable for producing a three dimensional workpiece using, e.g., paper, plastic, metal, or ceramic material. On the contrary, the CAM-LEM process disclosed in U.S. Pat. No. 5,779,833 is particularly suitable for producing inorganic green article.
The CAM-LEM process involves manufacturing thin sheets in an equipment (for example, a tape casting machine), and then cutting the sheets into cross sectional shapes of a workpiece using laser cutting equipment. Next, the sheets of the workpiece are laminated using a binder, followed by drying. Laminating processes are then performed repeatedly to form a three dimensional stack. Finally, the three dimensional stack is compressed through a press to enable the layers of the three dimensional stack to be in close contact with one another so as to bind them together in the subsequent sintering process. Therefore, the CAM-LEM process is complicated and is required to be performed at different working stations. Moreover, a plurality of equipments are also required in the CAM-LEM process.
The LOM process involves manufacturing thin sheets in an equipment (for example, a tape casting machine), and then adhering the sheets on a worktable, followed by cutting by laser. Although the LOM process requires less equipments and working stations, stacking and bonding the thin sheets is also required. Whether or not the bonding between the thin sheets is good may affect the microstructure of the green article and the sintering strength.
In U.S. Pat. No. 5,779,833, the thin sheets are bound by an adhesive layer disposed between the thin sheets or using a solvent to dissolve a binder in the thin sheets as an adhesive, and are pressed and heated to force the thin sheets to bind firmly. However, the adhesive layer is formed in the laminate thus made, and the microstructure of a green article made from the laminate may be non-uniform. Moreover, although use of the solvent may not change the composition of the green article and no adhesive layer is required, a longer time is necessary to dissolve the binder in the dry and hard thin sheet, and the binding strength cannot be properly controlled.
Since the thin sheets may not be in close contact by using the abovementioned processes, it is suggested that the stack of the thin sheets be further compressed in the CAM-LEM process. In this case, press and die are required. In addition, it is well known to a person skilled in the art that a stress gradient would be generated due to compression, thereby resulting in a non-uniform density. Although a more uniform density of the green article may be obtained by means of isostatic pressing, performing the isostatic pressing may increase equipment costs. In addition, since a die is required for the isostatic pressing, it is relatively inconvenient to provide a die specific for a workpiece with a complicated shape or fine features. A solution for the problem is to use a sacrificial material having a complementary cross section for the workpiece to form a simple square shape. Thus, a die used in the compressing process may have a simple square shape. The sacrificial material and the workpiece may be separated from each other after the compressing process is performed. However, the die is also required to be used in this process.
In view of the abovementioned CAM-LEM process, not only a large number of equipments are required, but also dies for the compressing step are required at a plurality of processing stations. Therefore, it is desirable to reduce the number of the required equipments, and to improve the bonding strength of the thin sheets without performing a compressing step and using a die.